Fire-resistant products are continually being sought for use in the building industry due to their obvious benefits of reducing the building occupants' risk of injury during fires. The theory behind the use of fire-resistant building materials is, of course, that such materials afford the building occupants more time to safely evacuate a burning building. While buildings typically now include a batting of mineral wool (e.g., glass wool) in interior wall cavities for the purpose of providing acoustical insulating properties, what has been needed is a mineral wool insulation which not only provides normal acoustical insulation properties, but which also is fire-resistant. It is towards the achievement of such a product that the present invention is directed.
According to this invention, it has been discovered that when an effective amount of a phosphate-containing compound (to be defined later) is brought into close proximity and/or contact with the surface of a mineral fiber (also to be defined later), and when the thus treated mineral fiber is exposed to temperatures well in excess of those which would normally melt an untreated fiber (e.g., temperatures in excess of about 649.degree. C. (1200.degree. F.)), the fiber nonetheless surprisingly withstands such elevated temperatures for significant time periods without melting.
What has been found according to the invention is that certain phosphate-containing compounds will form a protective high-temperature coating or layer on the mineral fiber surfaces when the treated fiber (i.e. a fiber having the phosphate-containing compounds in close proximity to and/or in contact with the fiber's surface) is exposed to thermal reaction conditions (i.e., elevated temperatures greater than about 300.degree. C. (572.degree. F.)).
Although not wishing to be bound to any particular theory, it is surmised that some of the phosphate-containing compounds which may be employed in the present invention are those which release phosphoric acid upon thermal degradation. This released phosphoric acid is believed to migrate to the surfaces of mineral fibers in close proximity and/or contact therewith where it reacts with the silica constituent of the glass to form a protective silicate phosphate ceramic coating or layer on the mineral fiber surfaces.
In fact, for certain phosphate-containing compounds, this migration phenomenon has been observed to, in effect, spread the protective ceramic coating to mineral fibers not actually in contact with, but in sufficiently close proximity to, the phosphate-containing compounds. It is this protective ceramic coating (as confirmed by X-ray diffraction analysis) that apparently renders the mineral fibers, mineral wool and mats of the invention capable of surprisingly withstanding the temperatures of an open flame for a significant period of time. That is, the mineral fibers, mineral wool and mats, and like products of this invention do not "burn through" when subjected to an open flame. Thus, the products of this invention exhibit "fire resistant" properties--that is, the products of this invention do not melt when exposed to open flame (i.e., temperatures normally melting untreated mineral fiber products). This property of the present invention is to be contrasted with "fire-retardant" properties--that is, products which merely inhibit flame spread, but do not necessarily resist flame burn through.
It is also surmised that phosphorous compounds which do not release phosphoric acid upon thermal degradation, offer protection of mineral fibers via formation of a high melting temperature phosphate surface coating or layer. In the case of some phosphate-containing compounds, it may be possible that a silicate phosphate ceramic and/or a high melting temperature phosphate coating is formed so as to impart fire-resistance properties to the mineral fibers.
The phosphate-containing compound can be brought into close proximity and/or contact with mineral fibers in any convenient manner, such as roll coating, spraying, dipping, sprinkling or padding. For example, when the mineral fibers are in the form of a glass wool, an aqueous mixture of the phosphate-containing compound may conveniently be sprayed via a conventional spray ring onto the glass fibers prior to their collection on a conveyor to form the glass wool. Also, in the particular case of glass wool, the phosphate-containing compound may be applied onto the glass fibers concurrently with an aqueous glass wool binder solution, in which case, the phosphate-containing compound is added to the binder solution prior to application.
In a particularly preferred technique, mineral wool (e.g., glass or rock wool) is bisected at or near its midplane after formation to form two mineral wool layers of substantially equal thicknesses between which a septum or substrate carrying the phosphate-containing compound is interposed. Thus, when the layers are brought into contact with respective surfaces of the septum, a composite "sandwich" structure is formed comprised of the two mineral wool layers and an interlayer comprised of the treated septum. A particularly preferred septum is a wet-laid nonwoven glass mat which is treated with the phosphate-containing compound.